1. Field of the Invention
This invention relates to the growth of large size or diameter, high quality, semiconductor-grade, single crystals of silicon carbide or other materials for optical and electronic device applications.
2. Description of the Prior Art
Single crystals of materials such as silicon carbide (SiC), at least in large sizes or diameters, are often grown in a physical vapor transport (PVT) reactor. All PVT reactors are cylindrical assemblages employing either induction or resistance heating positioned at an outer perimeter of a reaction chamber or reactor. By adjusting the vertical position and power of the heater, an axial temperature gradient is created which causes the sublimation and transport of a material vapor from a material source at the bottom of the reactor to a seed crystal at the top of the reactor. This process forms a single crystal of material, such as SiC, at the top of the reactor. Prior art patents in this area include, for example, U.S. Pat. Nos. 5,683,507; 5,611,955; 5,667,587; 5,746,827; and Re. 34,861.
This known cylindrical reactor design, which heats radially using a cylindrical susceptor/heater, relies on controlled heat losses from the cylinder to create the desired gradient to drive the growth process. These conventional cylindrical PVT reactors create a combination of detrimental radial and axial temperature gradients. This causes numerous problems in the growth of SiC or other crystal materials, including:
creation of thermal gradients and large thermal stresses in the material boules during growth and during cool-down, leading to the introduction of dislocations and cracking;
hindering the control of uniform crystal growth rates over time;
non-uniform source sublimation; and
severe limitations in scaling up the crystal site beyond the 3-4xe2x80x3 diameters to sizes that allow substantial wafer cost reduction.
It is an object of the sent invention to overcome these problems of the prior art designs.
Accordingly, we have invented an apparatus for producing large size, single crystals in a physical vapor transport reactor. The apparatus includes a reaction chamber having an axis therethrough and configured to receive a source material at a first end and a seed crystal at a second end of the reaction chamber spaced from and opposite the first end. A planar first heating element is positioned outside the reaction chamber, perpendicular to the axis of the reaction chamber and adjacent the source material. In this way, the first heating element is configured to heat the first end of the reaction chamber which receives the source material. Similarly, a planar second heating element is positioned outside the reaction chamber, perpendicular to the axis of the reaction chamber and adjacent the seed crystal. In this manner, the second heating element is configured to heat the second end of the reaction chamber which receives the seed crystal. A heat loss prevention means surrounds the reaction chamber and the first and second heaters for minimizing radial heat loss from the reaction chamber. A growth chamber surrounds the reaction chamber, the first and second heaters and the heat loss prevention means. A heat controller is provided for heating the first and second heating elements to different temperature levels so as to maintain a temperature drop from the first heating element to the second heating element. A vacuum controller is used to establish a desired vacuum in the growth chamber. In this manner, the heat controller and the vacuum controller are operable to provide a uniaxial heat flow from the source material to the seed crystal and planar isotherms so as to establish temperature and vacuum conditions in the reaction chamber to permit high quality crystal growth from the source material to the seed crystal through physical vapor transport.
The first and second heating elements may be induction heating elements each having a susceptor adjacent the reaction chamber and an associated induction coil. The susceptors are both preferably positioned within the growth chamber, although the induction coils of the induction heating elements may be positioned either within or outside the growth chamber. It is also possible to use resistance heaters for the first and second heating elements.
In one embodiment, the heat loss prevention means includes insulation material surrounding the reaction chamber and the first and second heating elements adjacent the reaction chamber. It is also possible to include as a heat loss prevention means a guard heater which surrounds the reaction chamber and extends axially along the length of the reaction chamber. In this embodiment, insulation material would surround the ends of the reaction chamber and the first and second heating elements.
The apparatus can also include a means for cooling the growth chamber. In one embodiment, the growth chamber would include spaced end plates at each end thereof and with cooling liquid, such as cooling water, circulating through the spacing between the spaced end plates at each end.
The heat controller can maintain the first heating element at a temperature of between 2050 and 2300xc2x0 C., maintain the second heating element at a temperature of between 2000 and 2250xc2x0 C. and maintain a temperature difference between the heating elements of between 5 and 150xc2x0 C. In addition, the heat controller can maintain the temperature of the heating elements so as to establish a thermal gradient between the seed crystal and the source material of between 5 and 50xc2x0 C./cm.
We have also developed a method for producing large size, single crystals in a physical vapor transport reactor. The method includes the steps of placing a quantity of source material at a first location in a reaction chamber, placing a seed crystal in the reaction chamber at a second location therein and spaced from the source material, placing the first as heating element outside the reaction chamber and adjacent the source material, placing a second heating element outside the reaction chamber and adjacent the seed crystal, placing the reaction chamber, with the source material and seed crystal therein, and the first and second heating elements, in a growth chamber, evacuating the reaction chamber to levels suitable for crystal growth, and heating the first and second heating elements to sufficient levels suitable for crystal growth and maintaining a temperature difference between the first heating element and the second heating element to provide a uniaxial heat flow within the reaction chamber from the source material to the seed crystal and to provide planar isotherms so as to permit high quality crystal growth from the source material to the seed crystal through a physical vapor transport process.
Both the apparatus of our invention and the method are particularly suitable for growing SiC from a source material and a seed crystal that are SiC.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.